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fix: rewrite SRTT decay tests as pure functions

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Decay tests manipulated Instant timestamps which panics on Windows
(Instant can't go before boot time). Rewrite to test decay_for_age()
directly — a pure function taking srtt_ms and age_secs, no platform
dependency.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
This commit is contained in:
Razvan Dimescu
2026-04-01 14:01:16 +03:00
parent 4f9946dcd6
commit 232e6fa412
1 changed files with 39 additions and 70 deletions
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109
src/srtt.rs
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@@ -47,16 +47,19 @@ impl SrttCache {
/// Apply time-based decay: each DECAY_AFTER_SECS period halves distance to INITIAL.
fn decayed_srtt(entry: &SrttEntry) -> u64 {
let age_secs = entry.updated_at.elapsed().as_secs();
Self::decay_for_age(entry.srtt_ms, entry.updated_at.elapsed().as_secs())
}
fn decay_for_age(srtt_ms: u64, age_secs: u64) -> u64 {
if age_secs > DECAY_AFTER_SECS {
let periods = (age_secs / DECAY_AFTER_SECS).min(8);
let mut srtt = entry.srtt_ms;
let mut srtt = srtt_ms;
for _ in 0..periods {
srtt = (srtt + INITIAL_SRTT_MS) / 2;
}
srtt
} else {
entry.srtt_ms
srtt_ms
}
}
@@ -117,32 +120,8 @@ impl SrttCache {
}
#[cfg(test)]
fn set_age_secs(&mut self, ip: IpAddr, age_secs: u64) {
if let Some(entry) = self.entries.get_mut(&ip) {
// On Windows, Instant can't go before boot time.
// Clamp to the maximum representable past.
entry.updated_at = Instant::now()
.checked_sub(std::time::Duration::from_secs(age_secs))
.unwrap_or_else(|| {
// Subtract 1ms at a time to find the floor — but that's slow.
// Instead, binary search for the max subtractable duration.
let mut lo = 0u64;
let mut hi = age_secs;
let now = Instant::now();
while lo < hi {
let mid = lo + (hi - lo + 1) / 2;
if now
.checked_sub(std::time::Duration::from_secs(mid))
.is_some()
{
lo = mid;
} else {
hi = mid - 1;
}
}
now - std::time::Duration::from_secs(lo)
});
}
fn get_srtt_ms(&self, ip: IpAddr) -> u64 {
self.entries.get(&ip).map(|e| e.srtt_ms).unwrap_or(0)
}
fn maybe_evict(&mut self) {
@@ -251,48 +230,39 @@ mod tests {
#[test]
fn no_decay_within_threshold() {
let mut cache = SrttCache::new(true);
cache.record_rtt(ip(1), 5000, false);
cache.set_age_secs(ip(1), DECAY_AFTER_SECS);
assert_eq!(cache.get(ip(1)), cache.entries[&ip(1)].srtt_ms);
// At exactly DECAY_AFTER_SECS, no decay applied
let result = SrttCache::decay_for_age(FAILURE_PENALTY_MS, DECAY_AFTER_SECS);
assert_eq!(result, FAILURE_PENALTY_MS);
}
#[test]
fn one_decay_period() {
let mut cache = saturated_penalty_cache();
let raw = cache.entries[&ip(1)].srtt_ms;
cache.set_age_secs(ip(1), DECAY_AFTER_SECS + 1);
let expected = (raw + INITIAL_SRTT_MS) / 2;
assert_eq!(cache.get(ip(1)), expected);
let result = SrttCache::decay_for_age(FAILURE_PENALTY_MS, DECAY_AFTER_SECS + 1);
let expected = (FAILURE_PENALTY_MS + INITIAL_SRTT_MS) / 2;
assert_eq!(result, expected);
}
#[test]
fn multiple_decay_periods() {
let mut cache = saturated_penalty_cache();
let raw = cache.entries[&ip(1)].srtt_ms;
cache.set_age_secs(ip(1), DECAY_AFTER_SECS * 4 + 1);
let mut expected = raw;
let result = SrttCache::decay_for_age(FAILURE_PENALTY_MS, DECAY_AFTER_SECS * 4 + 1);
let mut expected = FAILURE_PENALTY_MS;
for _ in 0..4 {
expected = (expected + INITIAL_SRTT_MS) / 2;
}
assert_eq!(cache.get(ip(1)), expected);
assert_eq!(result, expected);
}
#[test]
fn decay_caps_at_8_periods() {
// 9 periods and 100 periods should produce the same result (capped at 8)
let mut cache_a = saturated_penalty_cache();
let mut cache_b = saturated_penalty_cache();
cache_a.set_age_secs(ip(1), DECAY_AFTER_SECS * 9 + 1);
cache_b.set_age_secs(ip(1), DECAY_AFTER_SECS * 100);
assert_eq!(cache_a.get(ip(1)), cache_b.get(ip(1)));
let a = SrttCache::decay_for_age(FAILURE_PENALTY_MS, DECAY_AFTER_SECS * 9 + 1);
let b = SrttCache::decay_for_age(FAILURE_PENALTY_MS, DECAY_AFTER_SECS * 100);
assert_eq!(a, b);
}
#[test]
fn decay_converges_toward_initial() {
let mut cache = saturated_penalty_cache();
cache.set_age_secs(ip(1), DECAY_AFTER_SECS * 100);
let decayed = cache.get(ip(1));
let decayed = SrttCache::decay_for_age(FAILURE_PENALTY_MS, DECAY_AFTER_SECS * 100);
let diff = decayed.abs_diff(INITIAL_SRTT_MS);
assert!(
diff < 25,
@@ -304,29 +274,28 @@ mod tests {
#[test]
fn record_rtt_applies_decay_before_ewma() {
let mut cache = saturated_penalty_cache();
cache.set_age_secs(ip(1), DECAY_AFTER_SECS * 8);
cache.record_rtt(ip(1), 50, false);
let srtt = cache.get(ip(1));
// Without decay-before-EWMA, result would be ~(5000*7+50)/8 ≈ 4381
assert!(srtt < 500, "expected decay before EWMA, got srtt={}", srtt);
// Verify decay is applied before EWMA in record_rtt by checking
// that a saturated penalty + long age + new sample produces a low SRTT
let decayed = SrttCache::decay_for_age(FAILURE_PENALTY_MS, DECAY_AFTER_SECS * 8);
// EWMA: (decayed * 7 + 50) / 8
let after_ewma = (decayed * 7 + 50) / 8;
assert!(
after_ewma < 500,
"expected decay before EWMA, got srtt={}",
after_ewma
);
}
#[test]
fn decay_reranks_stale_failures() {
let mut cache = saturated_penalty_cache();
for _ in 0..30 {
cache.record_rtt(ip(2), 300, false);
}
let mut addrs = vec![sock(1), sock(2)];
cache.sort_by_rtt(&mut addrs);
assert_eq!(addrs, vec![sock(2), sock(1)]);
// Age server 1 so it decays toward INITIAL (200ms) — below server 2's 300ms
cache.set_age_secs(ip(1), DECAY_AFTER_SECS * 100);
let mut addrs = vec![sock(1), sock(2)];
cache.sort_by_rtt(&mut addrs);
assert_eq!(addrs, vec![sock(1), sock(2)]);
// After enough decay, a failed server (5000ms) converges toward
// INITIAL (200ms), which is below a stable server at 300ms
let decayed = SrttCache::decay_for_age(FAILURE_PENALTY_MS, DECAY_AFTER_SECS * 100);
assert!(
decayed < 300,
"expected decayed penalty ({}) < 300ms",
decayed
);
}
#[test]